THE ERUPTION OF VESUVIUS IN APRIL, 1906. 



243 



although of fine grain, is ahnost holocrystalline by a more or less 

 complete devitrification and larger development of the microlites. 

 The glass inclusions of the leucite are often transformed into augite 

 and titanomagnetite. 



From these observations it may be concluded that a part, at least, 

 of the phenocrysts of leucite are of intratelluric origin, but not of 

 great depth, because the lava poured out rapidly contains fewer of 

 them that the scorise thrown from the crater; furthermore, the 

 microlitic period of the leucite did not begin till the ejection. 



THE PRODUCTS OF THE VULCANIAN EXPLOSIONS. 



The most important of these products are those that have cov- 

 ered the cone and furnished the material for the dry avalanches. 

 They include the fine dust above described, the lapilli that fell at 

 Ottajano, and the material of the dry breccias. 



Lapilli — The greatest part of the lapilli is formed of black scori- 

 aceous material, either light with a glazed surface or heavy with a 

 tarnished surface, often reddish. They are distinguishable from the 

 material of the Strombolian explosions by large crystals of augite 

 and lamellie of ])iotite. Microscopic examination shows, further, a 

 few phenoci-ysts of leucite. plagioclase. apatite, and olivine, embedded 

 in a ground mass composed of augite needles, minute crystals of 

 leucite, and grains of magnetite. The chemical analysis of these 

 scorifie has gi\^n the following results, showing that they belong to 

 a type altogether different from that of the new lava in being less 

 aluminous, poorer in alkalies, but containing more magnesian and 

 more calcic : 



SiO, 48. 10 



ALOa 15.31 



Fe,03 

 FeO - 

 MgO 

 CaO 

 Na,0 



3.20 



5.45 



7. 55 



12. 45 



1.08 



KjO 



TiO, 



P,05 



Fire loss. 



100. 25 



The mineralogical and chemical study of these scoriae confirms 

 Avhat has been already said regarding their origin — that they have not 

 been derived from the new magma, but have been torn away from the 

 older rocks of the cone by the Vulcanian explosions. 



The lapilli are mixed with numerous fragments of nonscoriaceous 

 rocks, described below. 



Breccias. — The materials thrown upon the mountain slopes permit 

 of making an inventory of the rocks entering into the construction 

 of the cone of Vesuvius and its foundations and of studying the 



